iphone backup (64GB) onto iMAC (302GB free) says not enough space

I am trying to back up my 64gb iphone ( has 33gb free on the phone) using finder, onto my IMAC ( Big Sur, 16gbram 1tb HD, 3.3 gb i7 quad) with 305GB free and at the end of the backup it says it cannot do it because there is not enough space. I am eventually trying to install and use [decipher textmessage].

I HAVE NO IDEA HOW TO FIX THIS OR WHAT TO DO ...ANY IDEAS?